1. Field of the Invention
The present invention relates to high density, non-volatile memory. One aspect of the invention pertains to on-chip magnetic memory elements (sometimes known as MRAM). However, the general invention is believed applicable to a variety of magnetic memory configurations. This could include conventional integrated memory that is electronically selected. But further, it could include moving or moveable memory such as hard drives, floppy discs, and the like, as will become apparent with reference to the specification herein. More particularly, the present invention relates to the detection of unstable states in magnetic memory that could result in erroneous memory operation, e.g. affect accuracy of the data read from memory or cause reduced reliability of the memory. MRAM possibly can have non-movable or moveable implementations, as has been suggested in the literature. See, e.g., L. Richard Carley, Gregory R. Ganger, and David F. Nagle, MEMS-Based Integrated-Circuit Mass-Storage Systems. in COMMUNICATIONS OF THE ACM, November 2000, Vol.43, No.11, which is incorporated by reference herein.
2. Problems with the Art
Many types of storage technologies exist today. One type of storage technology or memory is magneto-resistive RAM or MRAM. MRAM includes various implementations including giant magneto-resistance (GMR) embodiments. Other examples of MRAM include, but are not necessarily limited to AMR, CMR, and SDT or TMR embodiments. MRAM has many desirable properties including random accessibility, very short write times, density close to dynamic RAM, sizes scaleable with lithography, very little cost, radiation hardness, and non-volatility.
A variety of other memory configurations and implementations exist, as are well known in the art. These include what will be referred to as xe2x80x9cmoving memoryxe2x80x9d, to differentiate it from conventional integrated memory that is electronically selected. One example is a hard drive, having a disc or other magnetic storage media that moves past a read and/or write head. Alternatively, the term xe2x80x9cmoving memoryxe2x80x9d will also refer to configurations where a read and/or write head moves past a magnetic memory, or both move relative to each other. There are also implementations being developed that utilize magnetic material, without necessarily any partitioned or segregated structure, as a storage location for data.
The concept of magneto-resistance is that when ferro-magnetic materials are subjected to a magnetic field, the electrical resistance can change. This is generally known as the magnetoresistance effect and may be observed to occur in many types of both single and composite films of magnetic materials. What makes this effect useful in a magnetic memory is that the magnetic orientation, or relative magnetic orientations of multiple layers may be set by manufacturing or writing of data and then sensed by way of this predictable resistance change. This is useful in both xe2x80x9cmoving memoryxe2x80x9d where the sensing element may be shared over many memory bits and in MRAM, where there is effectively one sense element per storage element. One popular type of MRAM makes use of a giant magneto-resistance (GMR) resistor. Publicly available literature currently discloses several other types, including a variety of implementations of GMR techniques and what are known as AMR, CMR, and SDT or TMR configurations. For purposes of illustration and example, magnetic memory in the form of a GMR resistor will be primarily discussed herein, but it is to be understood that the concepts discussed have applicability to the variety of magnetic memory types available, discussed, or contemplated in the art. It is believed the invention will also have applicability to many, if not all, future implementations of magnetic memory using magnetoresistive properties.
One implementation of a GMR resistor uses a pair of magnetic thin films having the configuration shown in FIG. 1. In FIG. 1, a GMR bit 10 is shown. The GMR bit 10 includes an upper layer 12 and a bottom layer 14. The upper and bottom layers are magnetic layers and are typically composed at least in part of permalloy. The upper layer 12 is sometimes designated as M1 and the lower layer 14 is sometimes designated as M2 to denote that these are magnetic layers. Between these magnetic layers 12 and 14 is an inner layer 16 which is nonmagnetic, and for the GMR case, is a conductive film such as copper. The magnetic layers 12 and 14 typically have magnetic moments aligned along an axis set during manufacturing but for this case we consider to be aligned either to the xe2x80x9cleftxe2x80x9d or to the xe2x80x9cright.xe2x80x9d
Examples of such magnetic devices, including GMR, tunneling devices, or other implementations, are described in the following publications and in the references cited in these publications, all of which are incorporated by reference herein:
(a) W. C. Black, Jr. and B. Das, xe2x80x9cProgrammable logic using giant-magnetoresistance and spin-dependent tunneling devicesxe2x80x9d, Journal of Applied Physics, Vol. 87, No. 9, Parts 2 and 3, May 1, 2000, pp. 6674-6679;
(b) R. Zhang, M. Hassoun, W. J. Black, Jr., B. Das, K. Wong, xe2x80x9cDemonstration of A Four State Sensing Scheme For A Single Pseudo-Spin Valve GMR Bitxe2x80x9d, IEEE Transactions on Magnetics, September 1999;
(c) W. C. Black, Jr. and M. Hassoun, U.S. Pat. No. 6,317,359, xe2x80x9cNon-Volatile Magnetic Circuitxe2x80x9d;
(d) W. C. Black, Jr., B. Das, M. Hassoun, U.S. Pat. No. 6,343,032, Non-Volatile Spin Dependent Tunnel Junction Circuit.
There are varieties of MRAM cells. For example, the bits may be of a xe2x80x9cspin-valvexe2x80x9d design where one of the layers is magnetically xe2x80x9cpinnedxe2x80x9d in one direction or a xe2x80x9cpseudo-spin-valvexe2x80x9d, where both layers are free to rotate but where one of the layers usually requires a greater field to switch its orientation.
Both spin valve and the pseudo-spin-valve varieties of memories have been used with various sensing schemes including non-destructive read out (NDRO) schemes. The advantage of non-destructive read out being that the memory may be read without changing the state in the memory. The current that runs through the GMR bit 10 as shown in FIG. 1 is commonly referred to as the sense current as it is used to determine or sense the state of the memory. The layer that runs either above, below or both above and below the GMR bit (shown in FIG. 1 as reference numeral 20) is commonly referred to as the word current as it is used to generate a controlled magnetic field to set the state of the memory during writing or produce a known magnetic field during reading. It should be noted that both sense and word line currents are important for setting the magnetic domains within the memory bit and this relationship is sometimes reflected in an xe2x80x9casteroid curvexe2x80x9d. By proper circuit biasing it may be possible to take advantage of this curve by using sense and word line currents together for X-Y addressing purposes, but there may also be additional current lines in the vicinity of the bit used for unique addressing purposes (sometimes known as xe2x80x98digitxe2x80x99 lines).
Different types of sensing schemes are known in the art. These prior art sensing methods include sequential sensing and dummy sensing. In sequential sensing, usually a word current is changed sequentially (such as applied in opposite directions sequentially) through the same GMR device, and the measured difference in resistance is compared to determine the state. For some types of memories it may be advantageous to switch direction of the sense current, or both the sense and word currents. See illustration of FIG. 2A.1 showing diagrammatically measuring R1 by sending Iword (xe2x88x92ev) through the word line relative the GMR device 54, and then in FIG. 2A.2, measuring R2 by sending Iword (+ev) through the word line. The memory states can be arbitrarily defined such that either or both layers may store data although one layer may also be used only for reading. The advantage of using one layer for reading is that non-destructive read-out schemes are easily implemented whereas use of both layers for data storage usually results in at least one layer being read-out destructively. An example of a four state sensing scheme can be found at R. Zhang, M. Hassoun, W. Black, Jr., B. Das, K. Wong, xe2x80x9cDemonstration of a Four State Sensing Scheme for a Single Pseudo-Spin Valve GMR Bitxe2x80x9d, IEEE Transactions on Magnetics, September 1999, incorporated by reference herein. For purposes of example and discussion, the bottom layer 14 (M2) can be used for xe2x80x9cstoragexe2x80x9d. Where M2 is used for storage of the data, the top layer 12 (M1) can be used for reading. If the magnetization direction of M2 is directed to the right, the state can be designated as xe2x80x9c0xe2x80x9d. If the magnetization direction of M2 is directed to the left, the state can be designated as xe2x80x9c1xe2x80x9d. Also, for the purposes of this example, we assume that the storage layer, M2, is magnetically harder than M1 (e.g. has a higher magnetic switching threshold.) Reading may be accomplished by establishing a sufficient magnetic field to place the layer M1 in a known direction but not so strong that it disturbs the orientation of layer M2. By then measuring the resistance (which we define as R(T1)) and then switching M1 to the opposite orientation and measuring the resistance again (giving R(T2)), we can infer the orientation of data storage layer M2. Note that this particular sensing scheme is NDRO (non destructive read-out), it is xe2x80x9cself-referencingxe2x80x9d, and does not require any additional reference or xe2x80x9cdummyxe2x80x9d bits to operate.
A high resistance is exhibited if the magnetic moments are anti-parallel, while a low resistance is exhibited if the magnetic moments of the two layers are parallel. Using this sequential sensing, and assuming that the first direction of M1 is to the left and the second direction of M1 is to the right, then if R(T1) is greater than R(T2), or that is to say if xcex94R=(R(T1)xe2x88x92R(T2)) is greater than 0, then the state is xe2x80x9c0xe2x80x9d. If R(T1) is less than R(T2), that is to say if xcex94R=(R(T1)xe2x88x92R(T2)) is less than 0, then the state is xe2x80x9c1xe2x80x9d. In this manner, the measured difference of the resistances is used to determine the state.
It should be noted that NDRO schemes similar to the above are also possible with xe2x80x9cspin-valvexe2x80x9d or magnetic devices with one magnetic layer xe2x80x9cpinnedxe2x80x9d in one direction. By using the unpinned layer for data storage and by then rotating it only slightly during reading (not enough to actually write the magnetically soft layer) a read scheme similar to that described immediately above can be achieved.
Another prior art sensing method is called dummy sensing. In dummy sensing there is a resistance associated with the bits used for storage designated as the real resistance, Rreal, and a resistance associated with a dummy resistance, Rdummy. See FIG. 2B, diagrammatically illustrating a GMR cell 54 and a separate xe2x80x9cdummyxe2x80x9d resistor 55 (e.g. another GMR cell or a poly resistor). The dummy resistance is designed to be between the resistance of a bit programmed as a xe2x80x980xe2x80x99 and one programmed as a xe2x80x981xe2x80x99. Dummy sensing is the comparing of the real resistor value with the dummy resistor value; in other words, xcex94R=(Rrealxe2x88x92Rdummy). The resulting xcex94R is then compared with 0 to determine the state of the GMR element. One configuration for dummy sensing is shown in FIG. 2B.
Despite the advantages of MRAM, including GMR elements, problems remain. These problems are not merely limited to GMR memory types but to other types of magnetic memory, including AMR, SDT, and others, including even movable media memories where data is stored magnetically on films that may move relative to the GMR or tunneling sense element. The problem that has not been fully addressed in the prior art is that the sensing schemes for these types of memories do not take into account unreadable or unstable states, which could, e.g., affect the measurement or determination of xcex94R, e.g. whether xcex94R greater than 0 or xcex94R less than 0 or some other static value. Unreadable states may be caused by many factors including untimely electrical or magnetic noise that results in an unacceptably small signal level and are not necessarily the result of damaged or defective magnetic memory. Hence, if this situation is detected, simply re-reading the bit or memory location may suffice if a non-destructive read-out scheme has been employed. This situation is sometimes known as a xe2x80x9csoft errorxe2x80x9d. Unstable magnetic states can also be caused by numerous factors but generally result from an undesired domain structure developing within the memory bits. This situation will generally not be resolved by re-reading and will probably result in unpredictable bit behavior until the bits are somehow repaired. These factors include surface roughness, noise, process variation, magnetic film inconsistencies, stray magnetic fields, self-heating or other temperature effects, and other factors. It should be noted that several methods exist for possible functional or actual repair of bits with unstable or undesirable domain structures including: (1) use of a redundant bit switched in so as to functionally appear to exist at the address of the affected bit or, (2) the use of large magnitude currents in one or more of the current lines through or in proximity of the bit to force the bit back into the desired single domain structure.
Therefore, it is a primary object, feature, or advantage of the present invention to improve upon the state of the art.
Another object, feature, or advantage of the present invention is to provide an improved memory sensing scheme.
Yet another object, feature, or advantage of the present invention is to provide an improved memory sensing scheme capable for use with magneto-resistive memory, including but not limited to MRAM.
It is a further object, feature, or advantage of the present invention to provide a memory sensing scheme capable of use with most, if not all, types of MRAM.
It is a still further object, feature, or advantage of the present invention to provide a sensing scheme that provides for the detection of unstable states in magnetic memory, including but not limited to MRAM.
Yet another object, feature, or advantage of the present invention is the provision of a sensing scheme that provides for reduced sensing error rate.
A further object, feature, or advantage of the present invention is a sensing scheme that provides for identification of read cycles with inadequate signal size.
A still further object, feature, or advantage of the present invention is the provision of a new sensing method for magnetic memory, including but not limited to MRAM, that is elegant in design.
Yet another object, feature, or advantage of the present invention is the provision of an improved sensing scheme for magnetic memory, including but not limited to MRAM, that is economical.
Other objects, features, or advantages of the present invention will become apparent from the following specification.
The invention includes methods, devices, and systems relating to magnetic memory and the sensing of states in magnetic memory, including the detection and/or identification of unstable states. One aspect of the invention relates to a method of sensing using a windowed sensing scheme and applied to an example of magnetic memory, namely MRAM. According to this method, unstable states may be detected so that a bit may either be electronically repaired or avoided in subsequent memory cycles. This method also allows for reduced sensing error rates by identifying read cycles with inadequate signal size caused by external noise or other effects. According to one aspect of the windowed sensing scheme, a threshold relative to the change in sensed resistance (herein referred to as xe2x80x9cdelta Rxe2x80x9d or xe2x80x9cxcex94Rxe2x80x9d) between first and second read states is defined. In the case of a single MRAM cell, delta R is the difference of resistance from either sequential or dummy sensing, as described previously. Alternatively, some other offset related to effective resistance of the memory element could be sensed, e.g., voltage or current. When the absolute value of xcex94R exceeds the threshold, the bit is considered to be in a stable state, but when the absolute value of xcex94R is less than the threshold, the sensed memory element is considered to be in an unstable state or affected by other parameters.
Detecting that the memory element is in an unstable state, provides a number of options. These include repairing the bit so that xcex94R exceeds the threshold, avoiding writing to the bit in the future, or otherwise identifying the cause of the unstable state or alleviating the effects of the unstable state.
Other aspects of the invention include applying this method to other types of magnetic memory. The xe2x80x9cstatexe2x80x9d of the magnetic memory element or xe2x80x9cbitxe2x80x9d is read by conventional methods, and that read state is compared to a reference threshold. The reference threshold is selected to insure that the signal that is read to indicate the state of the bit is indicative of a stable magnetic element and, if not, provide an indication from the read that there may be a problem, e.g. a problem with the element or that environmental conditions or other factors affected the element.
Another aspect of the invention involves a circuit for implementing a windowed sensing scheme. This circuit includes a reference generator and a window comparator. Optionally, the circuit may contain a repair circuit that may re-magnetize or re-establish the magnetic domains within the magnetic element so that xcex94R exceeds the threshold. In one aspect of the present invention, the windowed sensing scheme is implemented in a MRAM device such that unstable bits may be detected and optionally repaired.